A wide variety of bearings for rotating machinery, ranging from conventional bearings to noncontact bearings, are available. Conventional bearings, in which bearings physically contact a rotating device, are subject to many well known problems. These problems include frictional energy losses and mechanical wear.
Noncontact bearings, such as magnet bearings, overcome problems with friction and mechanical wear, but introduce other problems. For example, permanent magnet bearings are inherently unstable. As a result, they require external mechanical means to stabilize them in at least one degree of freedom. Electromagnet bearings, on the other hand, can be made inherently stable with position sensors and electronic feedback control loops. The electromagnets in the bearings, however, require a power source and a means for cooling their windings. As a result, electromagnet bearings can be impractical for many applications.
Superconductor magnetic bearings have been proposed as improvements to permanent magnet and electromagnet bearings. For example, U.S. Pat. Nos. 4,886,778 and 4,939,120, both to Moon et al., and commonly-owned U.S. Pat. No. 5,214,981, to Weinberger et al., describe several prior art superconductor bearings that use high temperature superconductors (HTS). In these bearings, which are assemblies of suitably arranged superconductor structures and permanent magnets, the superconductors and magnetic fields from the magnets interact to produce levitation forces. The interaction can be the result of the Meissner effect, magnetic flux pinning effects, or a combination of the two. Meissner effect forces are produced when a magnetic field of a permanent magnet is expelled by a superconductor from its interior. The magnitude of the Meissner effect forces is proportional to the fraction of the expelled magnetic flux. In weak magnetic fields, such as fields below the lower critical field (H.sub.c1), and at temperatures well below the critical temperature (T.sub.c), the superconductor can successfully expel all the external magnetic flux from its interior, except for the portion of the interior within a small distance (the London penetration depth) of its surface. If the dimensions of the superconductor are small with respect to the London penetration depth, the portion of the superconductor from which flux is excluded will be small compared to the portion of the superconductor in which flux penetration occurs. As a result, the Meissner levitation forces in such superconductors may be weak. Therefore, the dimensions of the superconductor should be large with respect to the penetration depth to obtain adequate levitation forces. At higher magnetic fields (e.g., H&gt;H.sub.c1), flux will penetrate beyond the London penetration depth in the form of discrete flux quanta. Under such circumstances, there may be reason to expand the dimensions of the superconductor even further.
The superconductor structures in superconductor bearings are usually made from bulk, polycrystalline monoliths of HTS material. Maximizing the levitation forced produced by a superconductor using simple Meissner effect levitation requires the exclusion of as much of an externally applied magnetic flux as possible from the superconductor. Cracks and grain boundaries in such materials, however, can allow magnetic fields to penetrate the material without flowing through crystalline grains. As a result, the effective penetration depth of a bulk HTS material may be many times its London penetration depth. Therefore, bulk materials ordinarily must be at least about 2 mm to about 8 mm thick (many times their London penetration depths) to achieve adequate levitation forces.
To overcome the problems created by cracks and grain boundaries in bulk HTS materials, thin film HTS materials have been proposed as substitutes. Thin film materials can be grown as single crystal, epitaxial films on a variety of substrates. Therefore, using simple Meissner effect levitation, HTS thin films can achieve levitation forces equivalent to those produced by much thicker bulk materials. In addition, HTS thin films can pin trapped vortices of magnetic flux more effectively than bulk materials of the same nominal composition. The superior flux pinning of HTS thin films can enhance the stiffness of the trapped flux, field cooled bearings described in commonly-owned, allowed U.S. patent application Ser. No. 07/791,834 by Weinberger et at. Bearing stiffness (.DELTA.F/.DELTA.z), a critical parameter in bearing design, is the restoring force generated per unit displacement of the bearing rotor and stator from their equilibrium positions. Moreover, epitaxial films may be crystallographically oriented to point a preferred axis in the direction of the applied magnetic field. Such orientation can further enhance the flux pinning capabilities of anisotropic superconductors, such as YBa.sub.2 Cu.sub.3 O.sub.7-.delta..
A further benefit of using HTS thin films in magnetic bearings may be the ability to use the geometric "demagnetization effect" to enhance levitation forces. Contrary to what the name of the effect implies, the demagnetization effect in HTS thin films amplifies the applied magnetic field and generates levitation forces that are appreciably larger than would be expected if the effect were ignored. FIG. 1 shows how this effect works. When a HTS thin film 2 is placed in an applied magnetic field (H.sub.a), H.sub.a induces a magnetization in the film. Because the Meissner effect in superconductors is diamagnetic, the induced magnetization (M.sub.i) is opposite to H.sub.a. As shown, the magnetic field lines from M.sub.i that flow between any segment 4 of the thin film 2 and any other segment 6 of the thin film reinforce H.sub.a because they approach the thin film in the same direction as H.sub.a. The field lines generated by segment 4, however, do not affect segment 4 itself. Rather, segment 4 is affected by induced field lines generated by other segments of the thin film 2. As a result, the total field to which the thin film 2 is exposed is much larger than H.sub.a alone. Therefore, the levitation force produced by the thin film 2 is larger than would be expected from considering only H.sub.a. The magnitude of the demagnetization effect is a function of the orientation and aspect ratio (length to thickness ratio) of the film 2 with respect to the direction of H.sub.a. The demagnetization effect provides its maximum benefit for a high aspect ratio thin film positioned perpendicular to H.sub.a as in FIG. 1. The geometric magnification of H.sub.a within the thin film 2 does not, by itself, result in leviation forces superior to those produced by bulk materials. Rather, the geometric magnification of H.sub.a in thin films produces equivalent forces with far less superconducting material and allows for the exploitation of the superior crystallinity, orientation, critical current, and flux pinning properties of HTS thin films. As a result, the size of a HTS thin film bearing can be reduced and the cryogenic engineering greatly simplified compared to a bearing made with a bulk HTS material.
As mentioned above, HTS thin films should be several times thicker than their London penetration depths to produce adequate levitation forces. This means they should be on the order of several microns thick. Because thin films lose epitaxial register with their substrates as they become thicker, however, it is extremely difficult to grow suitable films to thicknesses greater than about 1 .mu.m. As a result, thin films cannot easily be used to their full potential in superconductor magnetic bearings with prior art methods. Therefore, what is needed in the industry are thin film HTS bearings that produce larger levitation forces than can be achieved with prior art thin film bearings.